Hot water out of taps is not very hot.

you turn up the thermostat best way is call a plumber

First, I don't even want to know why someone would put a tankless outside as you have found out...really bad idea. In order to help you, more information is needed. Is this a gas or electric unit? Or a gas unit with electric controls? Is it mechanically or electronically controlled ( a make and model would help). Are the water lines to or from the tank exposed? When you say no hot water coming from the tap does that mean when you turn on the hot water faucet you just get cold water or no water?

There is an adjustment knob to control how hot the water can become. There might be a problem at the tap (however, if this is the case, you would still get hot water everywhere in the house). If the entire house isn't getting hot water, and the knob is set at the hottest water, you probably need to get the hot water tank replaced.

Gas or electric?
Electric water heater ... then test for crossover, check for water leak, replace thermostats, test elements
Gas water heater ... test for crossover, check for water leak
http://waterheatertimer.org/Crossover.html

http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

http://waterheatertimer.org/How-to-test-water-heater-element.html

water leak: turn off all taps and put ear against pipe and listen for water leak... check if water meter advances when all water taps are off.

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Hi, try turning up your temp setting in your rinnai. Also , it may take a few seconds to get hot water if the tap is quite a way from the hwc.

1) Thermostat may be bad, or out of calibration. Set temperature higher, and/or troubleshoot by type of gas control.
Copy following link to troubleshoot by type of gas control valve:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html


2) Dip tube broken
http://waterheatertimer.org/Water-heater-dip-tube.html

3) Water leak running hot water out of tank.
Turn off all taps and put ear against pipe listening for water leak.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

These Tankless water heaters attempt to heat water as it quickly travels through a short series of heated copper pipes. A high volume of water at an accelerated speed has little time to heat up as it passes through its heating source. You can turn up the temperature (which you undoubtably tried) and turn the water volume all the way down. These settings will produce the hottest temperature capable of the unit - given the entry temperature of your water source.
Most underground water enters at app 52 degrees - and becomes even colder due to ambient temperatures. Difficult to raise this temperature 60-70 degrees in a few seconds of heating.
I've routed a series of copper pipes adjesent to the home heating system in an attempt to pre-heat incomming water. It's only a few degrees warmer, but results a 20- 30 degree hotter delivery.
I have installed some top end units that simply delivers Hot water without compermise.

The problem with all that type of water heater is that once the hot tap is turned on sufficient flow must be maintained through the water heater to keep the main gas burner alight. If the heat adjuster is turned up to high, then the person using the hot water will turn it down to a low rate which means the main burner will turn off and result in cold water. Try setting the heat control to the lowest setting that still gives hot enough water. If you have a thermometer adjust control for a temperature of around 50 to 55 degrees Celcius. You may have to fiddle with fine adjustments to get the temperature and flow rates to maintain a constant output of hot water. You may still have intermittent problems. If the heater only supplies baths or showers you could set the temperatuer even lower (45 to 50) to ensure that the hot tap is used more than the cold which will maintain flow and therefore heat.

How big is your bathtub? Is your water heater set on the hottest setting? What is the temperature of the hot water at the tap?

Keep in mind that hot water can cause severe burns. Children and those with diminished mental capacity can be seriously harmed by hot water.

The "40 Gallon" rating on the water heater is usually the amount of hot water the unit can produce in one hour. So the tank, and the amount of immediately available hot water, might be smaller than 40 gallons.

My bathtub is 2 feet wide by 1 foot deep and 4 feet long. This is 8 cubic feet or almost 60 gallons. So a 40 gallons of water at 120 degrees, mixed with 20 gallons of cold water (60 degrees), fills the tub with 100 degree water. My January cold water is 42 degrees, so it will take 40 gallons at 129 degrees to fill the tub. If the tub is cold to start it will take even hotter water.

So the water heater might be just fine.

Possible solutions:
1. turn the temperature up on your water heater- or the temperature control could be malfunctioning and not getting the water hot enough.
2. insulate your hot water pipes so that the water loses less heat on the way to the tub.
3. bigger water heater.
4. smaller bathtub, or less full bathtub.

Try shutting off the gas, shut off the water, and drain the tank to remove any sediment that may interfere with heat transfer from the burner. If that doesn't do it, check the dip tube to make sure it is still intact. The dip tube directs incoming cold water to the bottom of the tank and the hot water is tapped from the top of the tank. If neither of those turn out to be the problem, check the thermo-couple. Since a gas water heater typically survives about 6-9 years, you may be better off just replacing it.